Make flash counter stop on 00:00:00:00?

[mrbrandonsmfof]

I can't figure out how to make my flash countdown stop on 00:00:00:00. It starts going into negative numbers. Help?

The code I'm using for my countdown is this:

this.onEnterFrame = function() {

var todayate = new Date();
var currentYear = today.getFullYear();
var currentTime = today.getTime();

var targetDateate = new Date(currentYear,3,21);
var targetTime = targetDate.getTime();

var timeLeft = targetTime - currentTime;

var sec = Math.floor(timeLeft/1000);
var min = Math.floor(sec/60);
var hrs = Math.floor(min/60);
var days = Math.floor(hrs/24);
sec = string(sec % 60);
if (sec.length < 2) {
sec = "0" + sec;
}
min = string(min % 60);
if (min.length < 2) {
min = "0" + min;
}
hrs = string(hrs % 24);
if (hrs.length < 2) {
hrs = "0" + hrs;
}
days = string(days);
if (days.length < 2) {
days = "0" + days;
}

var counter:String = days + " " + hrs + " " + min + " " + sec;

time_txt.text = counter;
}